The carpal tunnel is formed by an arch of the eight wrist bones, spanned on its palmar surface by the transverse carpal ligament, known as the flexor retinaculum. The carpal tunnel functions as a mechanical pulley to provide the appropriate moment arms for the digital flexor tendons as they pass through the tunnel. The tendons can then transmit force out into the fingers and impart only an appropriate amount of tension to develop torque at the level of the wrist.
Within the carpal tunnel, these tendons are lubricated and nourished by two synovial membranes--the radial and the ulnar bursa. The median nerve also shares the carpal tunnel, and then it branches out to provide sensory innervation to the palmar surface of the thumb, index, long and a portion of the ring finger. In addition, a small motor branch of the median nerve supplies the thenar muscles, which are responsible for lifting the thumb into opposition with the fingers.
Carpal tunnel syndrome refers to numerous clinical signs and symptoms resulting from pressure on the median nerve inside the carpal tunnel. The typical etiology is increased pressure within the carpal tunnel, which interferes with the function of the median nerve. The patient experiences numbness and tingling in the fingers, together with pain that may radiate as far as the shoulder or base of the neck. Other symptoms include impaired grasping ability (due to sensory deprivation from the digits), loss of sleep from pain and numbness in the hand, and weakness or atrophy of the thenar muscles.
The pathology generally results from a swelling of the synovial membranes, which is typically idiopathic. Carpal tunnel syndrome can also be caused by pressure on the median nerve from rheumatoid arthritis or edema in the final trimester of pregnancy, or other diseases.
Many instances of carpal tunnel syndrome can be treated conservatively, typically with a resting splint and cortisone injection into the carpal tunnel. However, if symptoms persist and/or reoccur, or if the patient has severe sensory deficit or loss of functions in the thenar muscles, then surgical decompression of the nerve by release of the transverse carpal ligament is often indicated.
Surgical decompression has been accomplished, in early techniques, by a longitudinal incision paralleling the thenar crease. The incision was carried down through the skin, subcutaneous fat, and palmar fascia to divide the palmaris brevis muscle and then the transverse carpal ligament. Most cases do not require any surgical treatment within the carpal tunnel, other than the division of the ligament. Thereafter, the skin is sutured and the patient was frequently splinted for about three weeks.
In the frequent cases where the syndrome is occupationally related, i.e., where workmen's compensation is involved, the patient was usually disabled for six to eight weeks. If the patient was a manual laborer, two or three months may pass before the return to gainful employment. This post-operative morbidity is primarily due to persistent tenderness in the palm as the scar tissue matures. Most patients experience tenderness in the heel of their hand for four to six months following the surgery.
Previously, a few surgeons would release the carpal tunnel by inserting scissors through a transverse incision proximal to the carpal tunnel. The blind release by division of the ligament would then proceed from the proximal to the distal end. When successful, this technique would decompress the median nerve (without scarring the heel of the patient's hand) and would significantly decrease postoperative pain and morbidity.
However, transverse incision and blind release is not advisable, due to the risk of incomplete release of the carpal tunnel, or injury to the superficial arterial arch and/or the median nerve. The superficial palmar arterial arch lies just distal to the distal portion of the transverse carpal ligament. The motor branch of the median nerve, which controls thumb opposition, is typically on the distal radial extent of the carpal tunnel, although anomalies can allow it to penetrate the transverse carpal ligament in any of a number of positions and be subject to injury during blind release procedures or surgical release by any method.
More recently, an improved surgical instrument and technique for effecting carpal tunnel release has been described in U.S. Pat. Nos. 4,963,147; 4,962,770; and 5,089,000. The instrument is also useful in other surgical techniques. The instrument includes a probe in which a cutting blade and an optical system are disposed. After the probe has been inserted into a body cavity, the cutting blade is extended through a lateral aperture in the probe to a position adjacent the selected tissue, while allowing the tissue manipulation to be observed.
The surgical instrument and technique described in the foregoing patents is useful in effecting complete division of the flexor retinaculum under visual inspection, while minimizing the risk of injury to surrounding tissue. In using the instrument, an incision is made at one end of the carpal tunnel, and then the probe is inserted through the incision and positioned adjacent to the flexor retinaculum. The cutting blade is then elevated and the probe is withdrawn so that the flexor retinaculum is divided, thereby releasing the carpal tunnel. The optical system enables continuous observation of the portion of the cutting blade within its field of view.
There are a number of potential disadvantages associated with complete division of the transverse carpal ligament, including alteration of the arch formed by the carpal bones, complete interruption of the pulley effect created by the transverse carpal ligament (allowing the digital flexor tendons and/or median nerve to sublux palmarwardly between the cut edges of the transverse ligament), and exposure of the cut edges of the transverse carpal ligament such that scar tissue necessary for its healing in the lengthened position is more abundant and therefore potentially creating greater post-operative morbidity, pain and weakness.
Furthermore, there is no objective data establishing that maximum lengthening of the transverse carpal ligament through its complete surgical division and subsequent reconstitution by scar tissue is necessary. For example, mechanical structures such as the Achilles' tendon were historically released by complete division only to discover by further study that optimal function could be obtained by selective lengthening of the contracted muscle tendon unit (the so-called Z-plasty lengthening).
There has not heretofore been provided an effective technique or surgical instrumentation to facilitate releasing the carpal tunnel without complete division of the flexor retinaculum (i.e., transverse carpal ligament).